In underground drilling, such as gas, oil or geothermal drilling, a bore is drilled through a formation deep in the earth. Such bores are formed by connecting a drill bit to a long assembly commonly referred to as a "drill string" that extends from the surface down to the drill bit. Drill strings are formed by connecting sections of drill pipe, which are typically made from steel or a composite material, typically in lengths of about 30 feet. The pipe sections are joined by threading the ends of the pipe sections so that a threaded male coupling on the end of one section screws into a threaded female coupling on the adjacent section, thereby forming a threaded joint.
As the drill bit advances into the earth, it forms the bore. In order to lubricate the drill bit and flush cuttings from its path, a high pressure fluid, referred to as "drilling mud," is directed through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through the annular passage formed between the drill string and the surface of the bore. Depending on the drilling operation, the pressure of the drilling mud flowing through the drill string internal passage will typically be between 1,000 and 20,000 psi. In addition, there is a large pressure drop at the drill bit 8. Consequently, the pressure of the drilling mud flowing through the annular passage (that is, outside of the drill string) may be 200 to 3,000 psi less than that of the pressure of the drilling mud flowing inside the drill string. Thus, a large pressure gradient acts radially across the joints joining adjacent sections of drill pipe. In addition to withstanding the pressure gradient, the joints between the drill pipe sections must also be sufficiently strong to withstand the torque, axial, and bending loads associated with the advancement and retraction of the drill bit. Consequently, the structural integrity of the joints cannot be compromised.
The distal end of the drill string, which includes the drill bit, is referred to as the "downhole assembly." In addition to the drill bit, the downhole assembly often includes specialized modules within the drill string that make up the electrical system for the drill string. Such modules may include sensing modules, a control module and a pulser module. The sensing modules may provide the drill operator with information concerning the formation being drilled through using techniques commonly referred to as "measurement while drilling" (MWD) or "logging while drilling" (LWD). Alternatively, the sensing modules may provide information concerning the direction of the drilling and can be used, for example, to control the direction in which the drill bit advances in a steerable drill string. Signals from the sensor modules are typically received and processed in the control module, which may direct the pulser modules to generate pulses within the flow of drilling fluid that contain information derived from the sensor signals. These pressure pulses are transmitted to the surface, where they are detected and decoded, thereby providing information to the drill operator.
As can be readily appreciated, electrically interconnecting the components of such an electrical system requires running electrical conductors through the drill pipe sections, which, in turn, requires that electrical connections be formed across the threaded joints coupling adjacent pipe sections.
In the past, such electrical connections have been made by installing contact rings in the radially extending faces of the pipe sections that come into electrical contact when the joint is fully assembled. One such approach is disclosed in U.S. Pat. No. 4,095,865 (Denison et al.). Unfortunately, this approach is unwieldy when multiple connections must be made across the same joint since there is insufficient space on the pipe section faces to permit multiple concentric contact rings. One approach for making multiple connections is to use a single ring with segmented contacts. However, this approach requires careful timing of the threading. Moreover, due to the environment associated with the drilling site, it is difficult to keep the contacts clean prior to and during assembly. Dirt and debris on the contact members can result in poor signal transmission.
Consequently, it would be desirable to provide an apparatus for making good quality electrical connections between multiple conductors across a joint coupling two tubular members, such as a joint in a drill string.